Tracking Hg2+ adsorption by reduced graphene oxide in continuous flow by in situ techniques

Mercury pollution poses severe risks to environmental and public health due to its high toxicity and persistence. The use of physical adsorbents for heavy metal cation uptake is a straightforward solution for many applications, but a better fundamental understanding of the mechanism is crucial for rational improvement. We have investigated the adsorption of mercury ions on reduced graphene oxide (rGO) in real time by coupling continuous flow setups with in situ analytical techniques: X-ray absorption spectroscopy (XAS) and electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D). The microfluidic setup provided a practical and efficient platform for mimicking realistic conditions, requiring minimal sample volumes and enabling continuous flow analysis, while the in situ XAS brought detailed atomic and electronic structural information, allowing for following changes in mercury ion coordination as adsorption proceeded. Similarly, EQCM-D followed the mass changes and viscoelastic properties of the rGO layer under dynamic flow conditions upon adsorption. The approach distinguished chemisorbed from physisorbed mercury cations, revealing a yet undescribed transition between the two forms. This enables a better understanding of the adsorption mechanisms and highlights the benefits of coupling microfluidic systems with advanced in situ techniques. • Real-time Hg²⁺ adsorption tracked using in situ XAS and EQCM-D techniques. • Chemisorbed and physisorbed Hg²⁺ species identified under flow conditions. • High Hg²⁺ uptake by rGO with fast kinetics and qₘₐₓ up to 410 mg·g⁻¹. • Microfluidic setup mimics realistic conditions with minimal sample volume.